The present invention relates to a toner supplying device for supplying toner to an electrostatic latent image developing unit for use in electrophotographic copying apparatus and the like.
Conventionally, such toner supplying devices are known as disclosed in Unexamined Japanese patent application No. 48-59847. The known device comprises a screw roller which is provided in a toner storage tank for transporting toner in the tank in a predetermined direction. The toner transported by the screw roller is supplied to a developing unit through an opening formed in the side wall of the storage tank near its bottom.
Such toner supplying devices adjust the amount of toner to be supplied to the developing unit according to the amount of rotation of the screw roller, so that these devices are widely used. However, the transport of toner in the predetermined direction by the screw roller, which is provided singly, displaces the toner toward the transport direction within the storage tank, with the result that the toner is inaccurately supplied.
In view of this, the present inventor discloses in Unexamined Japanese Utility Model application No. 57-63358 a device which comprises a first screw roller for transporting toner to an opening formed in the side wall of the toner storage tank close to the bottom thereof and a second screw roller for transporting the toner in a direction opposite to the direction of transport by the first screw roller. This device precludes the displacement of toner because the toner is circulated at the bottom of the storage tank.
However, in the toner supplying devices of both types described above, the opening provided in the storage tank for supplying toner is positioned at the lowermost part of the side wall of the tank. The toner is transported through this opening to a projecting member in the form of a pipe and then to the developing unit through an outlet of the projecting member.
While the projecting member is adapted to prevent the overflow of toner from the outlet in an amount greater than is transported by the screw rollers, the position of the projecting member is inevitably limited by the structure of the device, consequently limiting the position of the outlet and presenting difficulties in designing. More specifically, it becomes impossible to provide a toner storage tank of sufficient capacity owing to the position and configuration of the projecting member if the position of the developing unit or the space for the toner supplying device is limited.